I want to group some data being returned from a SQL 2012 database, I need to work out how to group by on a certain amount of fields.
The following SQL works fine
SELECT MessageId, SearchedString, COUNT(SearchedString) AS [SearchedStringCount], MAX(percentage) AS TopPercent
from (
select MessageId, SearchedString, Percentage
from table
where MessageId = '15'
) T
GROUP BY MessageId, SearchedString
But as soon as I add another fields in the select, then SQL is asking for it to be included in the group by, which isnt what i need.
How can I add another field to the above SQL, without having it be included in the Group By?
Ideally, I'm looking to include a Date value, like this:
select MessageId, SearchedString, COUNT(SearchedString) AS [SearchedStringCount], MAX(percentage) AS TopPercent, CAST(ScreenedDate AS DATE) AS DateScreened
from (
select MessageId, SearchedString, Percentage, ScreenedDate
from table
where MessageId = '15'
) T
GROUP BY MessageId, SearchedString
Simply by using an aggregate function like max and min(that is if you either want a random one or the column is always the same) like this:
select MessageId, SearchedString, COUNT(SearchedString) AS [SearchedStringCount], MAX(percentage) AS TopPercent, max(CAST(ScreenedDate AS DATE)) AS DateScreened
from (
select MessageId, SearchedString, Percentage, ScreenedDate
from table
where MessageId = '15'
) T
GROUP BY MessageId, SearchedString
When you use group by, all the columns(that are not aggregated) must be included in the group by section. So if you want another column to be included , there are several options depends on which results you want to get.
Option 1) the column is always the same, max or min should cover it.
Option 2) the column is different but it doesn't matter which one to take, max and min should cover that as well.
And option 3) You have to pick a specifiec one(like the latets or something) and then for each logic the answer will be different.
Using SQL Server you're obliged to:
Option 1: use aggregates in the SELECT list
or:
Option 2: add non-aggregated columns in the GROUP BY list
Other databases (for example MySQL) have what they call extended GROUP BY where they "... extend the standard SQL use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause...". In this case they return just "any value" for the non-aggregated column in the SELECT list not included in the GROUP BY. This makes sense only if you're sure that - in your query - ALL non-aggregated columns in the select list will return the same value.
With other databases (not SQL Server) I have coded a user defined function (any_value()) to return just the first retrieved value. It's useful, if pick option 1, because you don't have to waste CPU cycles looking for aggregates you don't need.
Related
I have a table as follows:
ParentActivityID | ActivityID | Timestamp
1 A1 T1
2 A2 T2
1 A1 T1
1 A1 T5
I want to select unique ParentActivityID's along with Timestamp. The time stamp can be the most recent one or the first one as is occurring in the table.
I tried to use DISTINCT but i came to realise that it dosen't work on individual columns. I am new to SQL. Any help in this regard will be highly appreciated.
DISTINCT is a shorthand that works for a single column. When you have multiple columns, use GROUP BY:
SELECT ParentActivityID, Timestamp
FROM MyTable
GROUP BY ParentActivityID, Timestamp
Actually i want only one one ParentActivityID. Your solution will give each pair of ParentActivityID and Timestamp. For e.g , if i have [1, T1], [2,T2], [1,T3], then i wanted the value as [1,T3] and [2,T2].
You need to decide what of the many timestamps to pick. If you want the earliest one, use MIN:
SELECT ParentActivityID, MIN(Timestamp)
FROM MyTable
GROUP BY ParentActivityID
Try this:
SELECT [ParentActivityId],
MIN([Timestamp]) AS [FirstTimestamp],
MAX([Timestamp]) AS [RecentTimestamp]
FROM [Table]
GROUP BY [ParentActivityId]
This will provide you the first timestamp and the most recent timestamp for each ParentActivityId that is present in your table. You can choose the ones you need as per your need.
"Group by" is what you need here. Just do "group by ParentActivityID" and tell that most recent timestamp along all rows with same ParentActivityID is needed for you:
SELECT ParentActivityID, MAX(Timestamp) FROM Table GROUP BY ParentActivityID
"Group by" operator is like taking rows from a table and putting them in a map with a key defined in group by clause (ParentActivityID in this example). You have to define how grouping by will handle rows with duplicate keys. For this you have various aggregate functions which you specify on columns you want to select but which are not part of the key (not listed in group by clause, think of them as a values in a map).
Some databases (like mysql) also allow you to select columns which are not part of the group by clause (not in a key) without applying aggregate function on them. In such case you will get some random value for this column (this is like blindly overwriting value in a map with new value every time). Still, SQL standard together with most databases out there will not allow you to do it. In such case you can use min(), max(), first() or last() aggregate function to work around it.
Use CTE for getting the latest row from your table based on parent id and you can choose the columns from the entire row of the output .
;With cte_parent
As
(SELECT ParentActivityId,ActivityId,TimeStamp
, ROW_NUMBER() OVER(PARTITION BY ParentActivityId ORDER BY TimeStamp desc) RNO
FROM YourTable )
SELECT *
FROM cte_parent
WHERE RNO =1
How can I use a Distinct or Group by statement on 1 field with a SELECT of All or at least several ones?
Example: Using SQL SERVER!
SELECT id_product,
description_fr,
DiffMAtrice,
id_mark,
id_type,
NbDiffMatrice,
nom_fr,
nouveaute
From C_Product_Tempo
And I want Distinct or Group By nom_fr
JUST GOT THE ANSWER:
select id_product, description_fr, DiffMAtrice, id_mark, id_type, NbDiffMatrice, nom_fr, nouveaute
from (
SELECT rn = row_number() over (partition by [nom_fr] order by id_mark)
, id_product, description_fr, DiffMAtrice, id_mark, id_type, NbDiffMatrice, nom_fr, nouveaute
From C_Product_Tempo
) d
where rn = 1
And this works prfectly!
If I'm understanding you correctly, you just want the first row per nom_fr. If so, you can simply use a subquery to get the lowest id_product per nom_fr, and just get the corresponding rows;
SELECT * FROM C_Product_Tempo WHERE id_product IN (
SELECT MIN(id_product) FROM C_Product_Tempo GROUP BY nom_fr
);
An SQLfiddle to test with.
You need to decide what to do with the other fields. For example, for numeric fields, do you want a sum? Average? Max? Min? For non-numeric fields to you want the values from a particular record if there are more than one with the same nom_fr?
Some SQL Systems allow you to get a "random" record when you do a GROUP BY, but SQL Server will not - you must define the proper aggregation for columns that are not in the GROUP BY.
GROUP BY is used to group in conjunction with an aggregate function (see http://www.w3schools.com/sql/sql_groupby.asp), so it's no use grouping without counting, summing up etc. DISTINCT eleminates duplicates but how that matches with the other columns you want to extract, I can't imagine, because some rows will be removed from the result.
The GROUP BY clause groups the rows, but it does not necessarily sort the results in any particular order. To change the order, use the ORDER BY clause, which follows the GROUP BY clause. The columns used in the ORDER BY clause must appear in the SELECT list, which is unlike the normal use of ORDER BY. [Oracle by Example, fourth Edition, page 274]
Why is that? Why does using GROUP BY influence the required columns in the SELECT clause?
Also, in the case where I do not use GROUP BY: Why would I want to ORDER BY some columns but then select only a subset of the columns?
Actually the statement is not entirely true as Dave Costa's example shows.
The Oracle documentation says that an expression can be used but the expression must be based on the columns in the selection list.
expr - expr orders rows based on their value for expr. The expression is based on
columns in the select list or columns in the tables, views, or materialized views in the
FROM clause. Source: Oracle® Database
SQL Language Reference
11g Release 2 (11.2)
E26088-01
September 2011. Page 19-33
From the the same work page 19-13 and 19-33 (Page 1355 and 1365 in the PDF)
http://docs.oracle.com/cd/E11882_01/server.112/e26088/statements_10002.htm#SQLRF01702
http://docs.oracle.com/cd/E11882_01/server.112/e26088/statements_10002.htm#i2171079
The bold text from your quote is incorrect (it's probably an oversimplification that is true in many common use cases, but it is not strictly true as a requirement). For instance, this statement executes just fine, although AVG(val) is not in the select list:
WITH DATA AS (SELECT mod(LEVEL,3) grp, LEVEL val FROM dual CONNECT BY LEVEL < 100)
SELECT grp,MIN(val),MAX(val)
FROM DATA
GROUP BY grp
ORDER BY AVG(val)
The expressions in the ORDER BY clause simply have to be possible to evaluate in the context of the GROUP BY. For instance, ORDER BY val would not work in the above example, because the expression val does not have a distinct value for each row produced by the grouping.
As to your second question, you may care about the ordering but not about the value of the ordering expression. Excluding unneeded expressions from the select lists reduces the amount of data that must actually be sent from the server to the client.
First:
The implementation of group by is one which creates a new resultset that differs in structure to the original from clause (table view or some joined tables). That resultset is defined by what is selected.
Not every SQL RDBMS has this restriction, though it is a always requirement that what is ordered by be either an aggregate function of the non-grouped columns (AVG, SUM, etc) or one of the columns grouped by, or functions upon more than one of those results (like adding two columns), because this is a logical requirement of the result of the grouping operation.
Second:
Because you only care about that column for the ordering. For example, you might have a list of the top selling singles without giving their sales (the NYT Bestsellers keeps some details of their data a secret, but do have a ranked list). Of course, you can get around this by just selecting that column and then not using it.
The data is aggregated before it is sorted for the ORDER BY.
If you try to order by any other column (that is not in the group by list or an aggregation function), what value would be used? There is no single value to use for ordering.
I believe that you can use combinations of the values for sorting. So you can say:
order by a+b
If a and b are in the group by. You just cannot introduce columns not mentioned in the SELECT. I believe you can use aggregation functions not mentioned in the SELECT, however.
Sample table
sample.grades
Name Grade Score
Adam A 95
Bob A 97
Charlie C 75
First Query using GROUP BY
Select grade, count(Grade) from sample.grades GROUP BY Grade
Output
Grade Count
A 2
C 1
Second Query using order by
select Name, score from sample grades order by score
Output
Bob A 97
Adam A 95
Charlie C 75
Third Query using GROUP BY and ordering
Select grade, count(Grade) from sample.grades GROUP BY Grade desc
Output
Grade Count
A 2
C 1
Once you start using things like Count, you must have group by. You can use them together, but they have very different uses, as I hope the examples clearly show.
To try and answer the question, why does group by effect the items in the select section, because that is what group by is meant to do. You can't do the count of a column if you do not group by that column.
Second question, why would you want to order by but not select all the columns?
If I want to order by the score, but do not care about the actual grade or even the score I might do
select name from sample.grades order by score
Output
Name
Bob
Adam
Charlie
Which results do you expect to see ordering by columns not listed in the select list and not participated in group by clause? at any case all kind of sort by non-mentioned in SELECT list columns will be omitted so Oracle guys added the restriction correctly.
with c as (
select 1 id, 2 value from dual
union all
select 1 id, 3 value from dual
union all
select 2 id, 3 value from dual
)
select id
from c
group by id
order by count(*) desc
Here my inderstanding
"The GROUP BY clause groups the rows, but it does not necessarily sort the results in any particular order."
-> you can use Group by without order by
"To change the order, use the ORDER BY clause, which follows the GROUP BY clause."
-> the rows are selected by defaut with primary key, and if you add order by you must add after group by
"The columns used in the ORDER BY clause must appear in the SELECT list, which is unlike the normal use of ORDER BY."
I have a table with 3 columns:
userid mac_address count
The entries for one user could look like this:
57193 001122334455 42
57193 000C6ED211E6 15
57193 FFFFFFFFFFFF 2
I want to create a view that displays only those MAC's that are considered "commonly used" for this user. For example, I want to filter out the MAC's that are used <10% compared to the most used MAC-address for that user. Furthermore I want 1 row per user. This could easily be achieved with a GROUP BY, HAVING & GROUP_CONCAT:
SELECT userid, GROUP_CONCAT(mac_address SEPARATOR ',') AS macs, count
FROM mactable
GROUP BY userid
HAVING count*10 >= MAX(count)
And indeed, the result is as follows:
57193 001122334455,000C6ED211E6 42
However I really don't want the count-column in my view. But if I take it out of the SELECT statement, I get the following error:
#1054 - Unknown column 'count' in 'having clause'
Is there any way I can perform this operation without being forced to have a nasty count-column in my view? I know I can probably do it using inner queries, but I would like to avoid doing that for performance reasons.
Your help is very much appreciated!
As HAVING explicitly refers to the column names in the select list, it is not possible what you want.
However, you can use your select as a subselect to a select that returns only the rows you want to have.
SELECT a.userid, a.macs
FROM
(
SELECT userid, GROUP_CONCAT(mac_address SEPARATOR ',') AS macs, count
FROM mactable
GROUP BY userid
HAVING count*10 >= MAX(count)
) as a
UPDATE:
Because of a limitation of MySQL this is not possible, although it works in other DBMS like Oracle.
One solution would be to create a view for the subquery. Another solution seems cleaner:
CREATE VIEW YOUR_VIEW (userid, macs) AS
SELECT userid, GROUP_CONCAT(mac_address SEPARATOR ',') AS macs, count
FROM mactable
GROUP BY userid
HAVING count*10 >= MAX(count)
This will declare the view as returning only the columns userid and macs although the underlying SELECT statement returns more columns than those two.
Although I am not sure, whether the non-DBMS MySQL supports this or not...
You cannot (should not) put non-aggregates in the SELECT line of a GROUP BY query.
I would however like access the one of the non-aggregates associated with the max. In plain english, I want a table with the oldest id of each kind.
CREATE TABLE stuff (
id int,
kind int,
age int
);
This query gives me the information I'm after:
SELECT kind, MAX(age)
FROM stuff
GROUP BY kind;
But it's not in the most useful form. I really want the id associated with each row so I can use it in later queries.
I'm looking for something like this:
SELECT id, kind, MAX(age)
FROM stuff
GROUP BY kind;
That outputs this:
SELECT stuff.*
FROM
stuff,
( SELECT kind, MAX(age)
FROM stuff
GROUP BY kind) maxes
WHERE
stuff.kind = maxes.kind AND
stuff.age = maxes.age
It really seems like there should be a way to get this information without needing to join. I just need the SQL engine to remember the other columns when it's calculating the max.
You can't get the Id of the row that MAX found, because there might not be only one id with the maximum age.
You cannot (should not) put non-aggregates in the SELECT line of a GROUP BY query.
You can, and have to, define what you are grouping by for the aggregate function to return the correct result.
MySQL (and SQLite) decided in their infinite wisdom that they would go against spec, and allow queries to accept GROUP BY clauses missing columns quoted in the SELECT - it effectively makes these queries not portable.
It really seems like there should be a way to get this information without needing to join.
Without access to the analytic/ranking/windowing functions that MySQL doesn't support, the self join to a derived table/inline view is the most portable means of getting the result you desire.
I think it's tempting indeed to ask the system to solve the problem in one pass rather than having to do the job twice (find the max, and the find the corresponding id). You can do using CONCAT (as suggested in Naktibalda refered article), not sure that would be more effeciant
SELECT MAX( CONCAT( LPAD(age, 10, '0'), '-', id)
FROM STUFF1
GROUP BY kind;
Should work, you have to split the answer to get the age and the id.
(That's really ugly though)
In recent databases you can use sum() over (parition by ...) to solve this problem:
select id, kind, age as max_age from (
select id, kind, age, max(age) over (partition by kind) as mage
from table)
where age = mage
This can then be single pass
PostgesSQL's DISTINCT ON will be useful here.
SELECT DISTINCT ON (kind) kind, id, age
FROM stuff
ORDER BY kind, age DESC;
This groups by kind and returns the first row in the ordered format. As we have ordered by age in descending order, we will get the row with max age for kind.
P.S. columns in DISTINCT ON should appear first in order by
You have to have a join because the aggregate function max retrieves many rows and chooses the max.
So you need a join to choose the one that the agregate function has found.
To put it a different way how would you expect the query to behave if you replaced max with sum?
An inner join might be more efficient than your sub query though.